Water-based adhesive for paper straw, and paper straw in which same is used

ABSTRACT

A water-based adhesive for a paper straw, comprising: an ethylene-vinyl alcohol copolymer (A); and water, wherein the ethylene-vinyl alcohol copolymer (A) has a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99. The water-based adhesive contains the ethylene-vinyl alcohol copolymer (A) having a specific structure, and is thus excellent in balance between the rapid coatability, the film strength, the initial adhesiveness, the dynamic water resistance, and the acid resistance, and particularly excellent in the rapid coatability, the film strength, and the acid resistance.

TECHNICAL FIELD

The present invention relates to a water-based adhesive for paper straws containing an ethylene-vinyl alcohol copolymer. Specifically, the present invention relates to a water-based adhesive for paper straws excellent in initial adhesiveness, dynamic water resistance, film hardness, rapid coatability, and acid resistance.

BACKGROUND ART

Conventionally, paper straws have been used as sundries, interior items, and materials to create room decoration items, artworks, and the like. However, due to the worsening of marine pollution by microplastics in recent years, eco-friendly paper straws for beverages attract attention as substitutes for plastic straws for beverages.

Paper straws today are often produced in the same method as that for general paper tubes. For example, paper straws are produced by applying an adhesive on base paper and then laminating and winding it on a metal rod in a spiral. As the adhesive, same as the conventional adhesives for paper tubes, starch, polyvinyl alcohols (hereinafter, may be referred to as PVAs), vinyl acetate emulsions, and the like are used.

Examples of conventionally reported adhesives for paper tubes include: an adhesive using a PVA obtained by saponifying a vinyl ester-based polymer using an acid catalyst (Patent Document 1); an adhesive made of clay containing a PVA and di- or higher valent metal salt of sulfuric acid (Patent Document 2), and these adhesives are improved in rapid coatability and initial adhesive strength. Patent Document 3 proposes an emulsion obtained by copolymerizing vinyl acetate monomers with N-methylolacrylamide using a PVA as a protective colloid to achieve high water resistance due to the crosslinking reaction of the N-methylolacrylamide monomer units.

In addition, to improve specific aspects of PVA performance, various modified PVAs are developed, and for example, Patent Document 4 proposes an aqueous emulsion, as an aqueous emulsion exhibiting high water resistance compared with conventional PVAs, comprising a PVA containing α-olefin units with a carbon number of 4 or less at a specific ratio as a dispersant and a (co)polymer using one or more monomers selected from unsaturated ethylenic monomers as a dispersoid. Moreover, Patent Documents 5 and 6 propose methods of emulsion (co)polymerizing vinyl acetate or vinyl acetate with (meth)acrylic ester using an ethylene-vinyl alcohol copolymer as a protective colloid. The resulting aqueous emulsions obtained by these methods are improved in water resistance and initial adhesiveness. Patent Document 7 describes an adhesive using an ethylene modified PVA for improvement in solution stability and water-resistant adhesiveness.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2-84478 A

Patent Document 2: JP 4-239085 A

Patent Document 3: JP 10-121017 A

Patent Document 4: JP 11-106727 A

Patent Document 5: JP 2000-119621 A

Patent Document 6: JP 2001-123138 A

Patent Document 7: JP 2001-172593 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Adhesives for paper straws are expected to have, in addition to the properties (e.g., rapid coatability, initial adhesiveness, film strength, water-resistant adhesiveness) expected from adhesives for paper tubes, properties specific to paper straw use. Examples of such a property include: dynamic water resistance capable of withstanding stirring a beverage with ice and the like; and acid resistance allowing use for low pH beverages, such as grape juice. The adhesives described in Patent Documents 1 and 2, however, have problems of solution stability and water-resistant adhesiveness and thus are not fully satisfied industrially. The emulsion described in Patent Document 3 has a problems of acid resistance, and because of low emulsion stability, has insufficient rapid coatability and film strength. The emulsions described in Patent Documents 4 through 6 have insufficient film strength and dynamic water resistance, and due to low emulsion stability, are also insufficient in rapid coatability. The adhesive described in Patent Document 7 has industrially dissatisfactory rapid coatability.

It is thus an object of the present invention to provide an adhesive for paper straws excellent in balance between rapid coatability, film strength, initial adhesiveness, dynamic water resistance, and acid resistance. It is also an object thereof to provide a paper straw satisfying the above properties.

Means for Solving the Problems

As a result of intensive studies, the present inventors have found that a water-based adhesive containing a specific ethylene-vinyl alcohol copolymer is capable of solving the above problems and have come to complete the present invention.

That is, the present invention relates to the following.

[1] A water-based adhesive for a paper straw, comprising: an ethylene-vinyl alcohol copolymer (A); and water, wherein

the ethylene-vinyl alcohol copolymer (A) has a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99.

[2] The adhesive according to [1] above, wherein the ethylene-vinyl alcohol copolymer (A) is dissolved in water. [3] The adhesive according to [1] or [2] above, wherein the ethylene-vinyl alcohol copolymer (A) has a viscosity-average degree of polymerization from 200 to 5000 and a degree of saponification from 80 to 99.7 mol %. [4] The adhesive according to any one of [1] through [3] above, wherein the adhesive is an aqueous emulsion containing a polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit. [5] The adhesive according to [4] above, wherein the polymer (B) contains 70 mass % or more of monomer units derived from at least one selected from the group consisting of a vinyl ester monomer, a (meth)acrylic ester monomer, a styrene monomer, and a diene monomer based on the total monomer units. [6] The adhesive according to [4] or [5] above, wherein the aqueous emulsion contains the ethylene-vinyl alcohol copolymer (A) as a dispersant. [7] The adhesive according to any one of [1] through [6] above, further comprising a PVA (C). [8] The adhesive according to [7] above, wherein the PVA (C) has a viscosity-average degree of polymerization from 200 to 5000 and a degree of saponification from 80 to 99.7 mol %. [9] The adhesive according to any one of [4] through [8] above, wherein a mass ratio (A)/(B) of the ethylene-vinyl alcohol copolymer (A) to the polymer (B) is from 2/98 to 80/20. [10] The adhesive according to any one of [1] through [9] above, further comprising an inorganic filler. [11] The adhesive according to any one of [1] through [10] above, further comprising from 0.000001 to 0.01 parts by mass of a compound with a molecular weight of 1000 or less having a conjugated double bond based on 100 parts by mass of the ethylene-vinyl alcohol copolymer (A). [12] A paper straw obtained by adhering a paper substrate to each other using the adhesive according to any one of [1] through [11] above. [13] A paper straw comprising an ethylene-vinyl alcohol copolymer (A), the ethylene-vinyl alcohol copolymer (A) having a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99.

Effects of the Invention

The water-based adhesive for paper straws of the present invention contains the ethylene-vinyl alcohol copolymer (A) having a specific structure and is thus excellent in balance between rapid coatability, film strength, initial adhesiveness, dynamic water resistance, and acid resistance and particularly excellent in rapid coatability, film strength, and acid resistance. In addition, the water-based adhesive has further improved initial adhesiveness and dynamic water resistance by adding the compound with a molecular weight of 1000 or less having a conjugated double bond or the inorganic filler thereto or by using the aqueous emulsion containing the ethylene-vinyl alcohol copolymer (A) as a dispersant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a polymerization apparatus used in Examples.

FIG. 2 is a schematic view of an exemplary wide paddle blade used in the present invention.

FIG. 3 is a schematic view of a three-roll coater used for evaluation of adhesives in Examples.

MODES FOR CARRYING OUT THE INVENTION

A water-based adhesive of the present invention is a water-based adhesive for paper straws, comprising: an ethylene-vinyl alcohol copolymer (A); and water, wherein the ethylene-vinyl alcohol copolymer (A) has a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99.

Ethylene-Vinyl Alcohol Copolymer (A)

The water-based adhesive of the present invention contains the ethylene-vinyl alcohol copolymer (A) characterized in having a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99. These points are described below.

Block Character of Ethylene Units

A major characteristic of the ethylene-vinyl alcohol copolymer (A) is that the block character of the ethylene units is from 0.90 to 0.99. The block character of 0.90 or more improves rapid coatability and initial adhesiveness of the water-based adhesive and strength of the film to be formed. The block character is preferably 0.93 or more and more preferably 0.95 or more. Meanwhile, the block character is 0.99 or less. The block character of 0.99 or less improves dynamic water resistance of the water-based adhesive and the strength of the film to be formed.

It should be noted that the block character is a numerical value that indicates the distribution of the ethylene units and vinyl alcohol units formed by saponification of vinyl ester units and takes values between 0 and 2. The value “0” indicates that the ethylene units or the vinyl alcohol units are distributed fully blockwise. With an increase in the value, both units are distributed more alternately. The value “1” indicates that the ethylene units and the vinyl alcohol units are completely randomly present. The value “2” indicates that the ethylene units and the vinyl alcohol units are completely alternately present. The block character is determined by ¹³C-NMR as follows. First, an ethylene-vinyl alcohol copolymer is saponified to a degree of saponification of 99.9 mol % or more, followed by thorough washing with methanol and drying at 90° C. under reduced pressure for 2 days. The completely saponified ethylene-vinyl alcohol copolymer thus obtained is dissolved in DMSO-d₆ and then the sample thus obtained is subjected to measurement using a 500 MHz ¹³C-NMR spectrometer (JEOL GX-500) at 80° C. Using a molar fraction of vinyl alcohol-ethylene two-unit chains (AE), a molar fraction of the vinyl alcohol units (A), and a molar fraction of the ethylene units (E) assigned and calculated from the obtained spectrum as described in T. Moritani and H. Iwasaki, 11, 1251-1259, Macromolecules (1978), a block character (η) of the ethylene units is determined according to the following equation:

η=(AE)/{2×(A)×(E)}

The ethylene-vinyl alcohol copolymer (A) having the block character of the ethylene units defined above can be produced by a special method comprising a polymerization step and a saponification step described later. This production method is described later in detail. The present inventors have found that a water-based adhesive excellent in balance between the rapid coatability, the film strength, the initial adhesiveness, the dynamic water resistance, and the acid resistance and particularly having high rapid coatability, film strength, and acid resistance is obtained using the ethylene-vinyl alcohol copolymer (A) with the block character of the ethylene units satisfying the range described above. The ethylene-vinyl alcohol copolymer (A) will be further detailed below.

Ethylene-Vinyl Ester Copolymer

The ethylene-vinyl alcohol copolymer (A) of the present invention is produced by copolymerizing ethylene and a vinyl ester to give an ethylene-vinyl ester copolymer and then saponifying the ethylene-vinyl ester copolymer. Examples of the vinyl ester to be used include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate, particularly preferably vinyl acetate.

Ethylene Unit Content

The ethylene-vinyl alcohol copolymer (A) of the present invention has a content of the ethylene units of 1 mol % or more and less than 20 mol %. If the ethylene unit content is less than 1 mol %, the resulting adhesive exhibits insufficient dynamic water resistance and rapid coatability. The ethylene unit content is preferably 1.5 mol % or more and more preferably 2 mol % or more. If the ethylene unit content is 20 mol % or more, the ethylene-vinyl alcohol copolymer is insoluble in water and preparation of an adhesive becomes difficult. The ethylene unit content is preferably 15 mol % or less, more preferably 10 mol % or less, and even more preferably 8.5 mol % or less.

The ethylene unit content is determined, for example, from ¹H-NMR of a precursor of an ethylene-vinyl alcohol copolymer or an ethylene-vinyl ester copolymer as a re-acetylated product by the following method. A sample of the ethylene-vinyl ester copolymer is purified by reprecipitation three or more times using a mixed solution of n-hexane and acetone and then dried at 80° C. under reduced pressure for 3 days to prepare an ethylene-vinyl ester copolymer for analysis. The ethylene-vinyl ester copolymer thus obtained is dissolved in DMSO-d₆, followed by ¹H-NMR (500 MHz) analysis at 80° C. The ethylene unit content can be calculated using peaks derived from the main-chain methine of the vinyl ester (4.7 to 5.2 ppm) and peaks derived from the main-chain methylene of ethylene and the vinyl ester (0.8 to 1.6 ppm).

Degree of Saponification

The ethylene-vinyl alcohol copolymer (A) of the present invention preferably has a degree of saponification of 80 mol % or more, more preferably 82 mol % or more, and even more preferably 85 mol % or more. Meanwhile, the degree of saponification is preferably 99.7 mol % or less, more preferably 99 mol % or less, and even more preferably 98.5 mol % or less. When the degree of saponification is the lower limit or more, transparency of an aqueous solution of the ethylene-vinyl alcohol copolymer (A) and the dynamic water resistance of the resulting adhesive is further improved. Meanwhile, when the degree of saponification is the upper limit or less, stable production of the ethylene-vinyl alcohol copolymer (A) is facilitated and polymerization stability is also improved because aggregates are less likely to be produced during preparation of an aqueous emulsion described later. The degree of saponification of the ethylene-vinyl alcohol copolymer (A) can be determined in accordance with JIS K6726 (1994).

Viscosity-Average Degree of Polymerization

The ethylene-vinyl alcohol copolymer (A) of the present invention preferably has a viscosity-average degree of polymerization of 200 or more, more preferably 250 or more, even more preferably 300 or more, and particularly preferably 400 or more. Meanwhile, the viscosity-average degree of polymerization is preferably 5000 or less, more preferably 4500 or less, even more preferably 4000 or less, and particularly preferably 3500 or less. When the viscosity-average degree of polymerization is the lower limit or more, the resulting water-based adhesive has excellent film formability and thus has further improved dynamic water resistance. Meanwhile, when the viscosity-average degree of polymerization is the upper limit or less, the water-based adhesive having dissolved ethylene-vinyl alcohol copolymer (A) has not too high viscosity and is readily handled. The viscosity-average degree of polymerization is referred to as P and can be determined in accordance with JIS K6726 (1994). That is, the ethylene-vinyl alcohol copolymer (A) of the present invention is re-saponified to a degree of saponification of 99.5 mol % or more and purified, and then P can be calculated by the following equation using limiting viscosity [η] (L/g) determined in water at 30° C.

P=([η]×10000/8.29)^((1/0.62))

Other Monomer Units

The ethylene-vinyl alcohol copolymer (A) of the present invention may contain monomer units other than the vinyl alcohol units, the ethylene units, and the vinyl ester units as long as the effects of the present invention are not impaired. Examples of such other monomer units include units derived from: α-olefins, such as propylene, n-butene, and isobutylene; acrylic acid and salts thereof; acrylic esters; methacrylic acid and salts thereof; methacrylic esters; acrylamides; acrylamide derivatives, such as N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamide propanesulfonic acid and salts thereof, acrylamide propyldimethylamine and salts thereof and quaternary salts thereof, and N-methylolacrylamide and derivatives thereof; methacrylamides; methacrylamide derivatives, such as N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamide propanesulfonic acid and salts thereof, methacrylamide propyldimethylamine and salts thereof and quaternary salts thereof, N-methylolmethacrylamide and derivatives thereof; vinyl ethers, such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles, such as acrylonitrile and methacrylonitrile; vinyl halides, such as vinyl chloride and vinyl fluoride; vinylidene halides, such as vinylidene chloride and vinylidene fluoride; allyl compounds, such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids, such as maleic acid, itaconic acid, and fumaric acid, and salts thereof and esters thereof; vinylsilyl compounds, such as vinyltrimethoxysilane; isopropenyl acetate; and the like. A content of these other monomer units is, depending on the intended objective, use, and the like, preferably 10 mol % or less, more preferably less than 5 mol %, even more preferably less than 1 mol %, and particularly preferably less than 0.5 mol %.

Method of Producing Ethylene-Vinyl Alcohol Copolymer (A)

A preferred method of producing the ethylene-vinyl alcohol copolymer (A) of the present invention is a method of producing the ethylene-vinyl alcohol copolymer (A) by reacting ethylene with the vinyl ester to give an ethylene-vinyl ester copolymer and then saponifying the ethylene-vinyl ester copolymer, comprising, in production of the ethylene-vinyl ester copolymer, (a) the step of contacting a solution containing the vinyl ester with an ethylene-containing gas while stirring the solution with a wide paddle blade in a polymerization vessel to obtain a stirring power Pv per unit volume from 0.5 to 10 kW/m³ and a Froude number Fr from 0.05 to 0.2. By contacting the solution containing the vinyl ester with the ethylene-containing gas in such a manner, the block character of the ethylene units of the ethylene-vinyl alcohol copolymer (A) can be within the above range. The production method will be detailed below.

Polymerization Step

In the polymerization step, ethylene is reacted (copolymerized) with the vinyl ester to give an ethylene-vinyl ester copolymer. The copolymerization of ethylene and the vinyl ester is preferably conducted by solution polymerization where ethylene and the vinyl ester are copolymerized in an organic solvent, such as an alcohol. Examples of the alcohol include lower alcohols, such as methanol and ethanol, particularly preferably methanol. Examples of an initiator used for the polymerization may be known initiators, such as azo initiators and peroxide initiators including 2,2′-azobis(isobutyronitrile), 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), benzoyl peroxide, n-propyl peroxydicarbonate, and the like.

In the polymerization, a chain transfer agent may coexist for adjustment of the viscosity-average degree of polymerization of the ethylene-vinyl ester copolymer thus obtained and the like. Examples of the chain transfer agent include aldehydes, such as acetaldehyde, propionaldehyde, butylaldehyde, and benzaldehyde; ketones, such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans, such as 2-hydroxyethanethiol; thiocarboxylic acids, such as thioacetic acid; halogenated hydrocarbons, such as trichloroethylene and perchloroethylene; and the like. Among all, aldehydes and ketones are preferably used. The amount of the chain transfer agent depends on the chain transfer constant of the chain transfer agent to be added and a targeted viscosity-average degree of polymerization of the ethylene-vinyl ester copolymer (A), and it is generally from 0.1 to 10 parts by mass based on 100 parts by mass of the vinyl ester to be used.

The polymerization may be carried out by any of continuous polymerization, semi-batch polymerization, and batch polymerization, preferably continuous polymerization. Examples of a polymerization reactor include a continuous tank reactor, a batch reactor, a tube reactor, and the like, preferably a continuous tank reactor.

With reference to the drawings, a specific polymerization apparatus and the polymerization step using the apparatus are described below. FIG. 1 is a schematic view of a polymerization apparatus used in Example 1. The apparatus is a continuous tank reactor in which a polymerization vessel 1 is connected to a heat exchanger 2 via pipes 3 and 4. In the heat exchanger 2, the vinyl ester can be brought into countercurrent contact with ethylene.

A plurality of pipes 5, 6, 7 are connected to the polymerization vessel 1. The number and arrangement of the pipes are not limited to those illustrated in FIG. 1. Ethylene, the polymerization initiator, and the organic solvent are fed to the polymerization vessel 1 through these pipes. The ratio of the materials introduced into the polymerization vessel per unit time is preferably from 0.1 to 20 parts by mass for ethylene, from 1 to 100 parts by mass for the organic solvent, and from 0.00001 to 1 part by mass for the polymerization initiator based on 100 parts by mass of the vinyl ester. In some cases, the vinyl ester and the other monomers can be fed through these pipes. The reaction liquid in the polymerization vessel 1 is continuously discharged from a reaction liquid outlet pipe 9 connected to the bottom of the polymerization vessel 1.

In the polymerization vessel 1, a stirrer 8 having a wide paddle blade is placed as a stirring blade. The solution containing the vinyl ester is caused to contact the ethylene-containing gas while stirred with the wide paddle blade, thereby reacting ethylene with the vinyl ester to give an ethylene-vinyl ester copolymer.

A wide paddle blade is preferably used as a stirring blade for stirring the solution containing the vinyl ester. FIG. 2 is a schematic view of an exemplary wide paddle blade used in the present invention. As illustrated in FIG. 2, the wide paddle blade is characterized in having a wide paddle with a width b. The width b of the wide paddle blade can be appropriately adjusted by the volume of the polymerization vessel 1 and the like, and is preferably from 1 to 10 m as described later. Use of such a paddle causes the solution to be homogeneously mixed from the bottom to the surface of the solution and also ethylene to be efficiently absorbed by the solution containing the vinyl ester. The wide paddle blade may be a single blade type (e.g., Maxblend blade) or a multi-blade type (e.g., Fullzone blade). In the light of further efficient absorption of ethylene by the vinyl ester, the surface level of the solution containing the vinyl ester is preferably near the upper end of the stirring blade while stirring the solution. Specific examples of the wide paddle blade include Maxblend blades (Sumitomo Heavy Industries Process Equipment Co., Ltd.), Fullzone blades (Kobelco Eco-Solutions Co., Ltd.), Sanmeler blades (Mitsubishi Heavy Industries, Ltd.), Hi-Fi mixer blades (Soken Chemical & Engineering Co., Ltd.), Supermix blade (Satake Chemical Equipment Mfg., Ltd., Supermix MR203, Supermix MR205), and Bendleaf blades (Hakko Sangyo Co., Ltd.).

An ethylene pressure in the polymerization vessel during a polymerization reaction is preferably from 0.01 to 0.9 MPa, more preferably from 0.05 to 0.8 MPa, and even more preferably from 0.1 to 0.7 MPa. A polymerization ratio of the vinyl ester at an outlet of a polymerization vessel is, but not limited to, preferably from 10% to 90% and more preferably from 15% to 85%.

The polymerization temperature is, but not limited to, preferably from 0° C. to 180° C., more preferably from 20° C. to 160° C., and even more preferably from 30° C. to 150° C.

While ethylene is reacted with a vinyl ester, the solution containing the vinyl ester is preferably stirred to have a stirring power Pv per unit volume from 0.5 to 10 kW/m³ in the polymerization vessel. If the stirring power is less than 0.5 kW/m³, the amount of ethylene incorporated in the vinyl ester is insufficient and the homogeneity of the reaction liquid is also insufficient, so that it is not possible to obtain the ethylene-vinyl alcohol copolymer (A) having a block character of the ethylene units within the above range. The stirring power is more preferably 1 kW/m³ or more and even more preferably 1.5 kW/m³ or more. If the stirring power is more than 10 kW/m³, power used for operation becomes very large, which is industrially unfavorable. The stirring power is more preferably 7 kW/m³ or less and even more preferably 5 kW/m³ or less. The stirring power Pv per unit volume for the solution containing the vinyl ester is determined by a method described later in Examples.

The Froude number Fr is a ratio of inertial force to gravity defined by the following equation, which is an indicator of the vortex shape in the liquid surface.

Fr=n ² ×d/g

n: rotation number (rps) of the stirring blade

d: stirring blade diameter (m)

g: gravity acceleration (m/s²)

While ethylene is reacted with the vinyl ester, the solution containing the vinyl ester is preferably stirred to have a Froude number Fr from 0.05 to 0.2 in the polymerization vessel. By adjusting Froude number Fr within the above range to control the vortex shape in the liquid surface, ethylene is moderately absorbed by the vinyl ester and it is thus considered that the ethylene-vinyl alcohol copolymer (A) having a block character of the ethylene units within the above range is readily obtained. The Froude number Fr is more preferably 0.06 or more and even more preferably 0.07 or more. The Froude number Fr is more preferably 0.18 or less and even more preferably 0.15 or less. The Froude number Fr may be controlled within the above range by appropriately varying the rotation number n or the diameter d of the stirring blade.

The stirring blade diameter d of the wide paddle blade is not particularly limited as long as the stirring power Pv and the Froude number Fr are adjusted within the above ranges, and it is preferably from 0.5 to 5 m in the light of improving ethylene absorption efficiency. The stirring blade diameter d is more preferably 0.75 m or more. The stirring blade diameter d is more preferably 4 m or less. The stirring blade diameter d is twice the distance from a rotating shaft to the distal end of the blade (the farthest point from the rotating shaft).

The width b (length in the height direction) of the wide paddle blade (paddle) is not particularly limited and may be adjusted to the volume of the polymerization vessel 1 and the like, and it is preferably from 1 to 10 m in the light of improving the ethylene absorption efficiency. The width b is more preferably 1.5 m or more. The width b is more preferably 8 m or less.

A ratio of the width (paddle width) b to the stirring blade diameter d (b/d) of the wide paddle blade is not particularly limited and may be determined depending to the shape of the polymerization vessel 1 or the like, and it is preferably 1 or more in the light of improving the ethylene absorption efficiency. The ratio (b/d) is generally 2.5 or less.

The shape of the polymerization vessel 1 is generally, but not limited to, almost columnar. In this case, the wide paddle blade is arranged in the almost columnar polymerization vessel 1 such that the rotating shaft of the wide paddle blade is aligned with the rotation axis of the polymerization vessel 1. A ratio (d/D) of the stirring blade diameter d (m) to an inner diameter D (m) of the polymerization vessel is not particularly limited as long as the effects of the present invention are not impaired. The ratio may be appropriately adjusted depending on the polymerization vessel to be used, and generally from 0.4 to 0.9. The volume of the polymerization vessel is generally, but not limited to, from 1 to 200 kl.

The rotation number n of the stirring blade is not particularly limited as long as the stirring power Pv and the Froude number Fr are adjusted within the above ranges, and it is preferably from 0.5 to 1.35 rps. If the rotation number n is less than 0.5 rps, the polymerization solution tends to be supercooled near a heat transfer surface, so that a gelatinous material is sometimes formed on the inner wall of the polymerization vessel, leading to difficulty in long-term operation. If the rotation number n is more than 1.35 rps, use of a low-viscosity polymerization solution sometimes causes the solution to splash and adhere to the inner wall of the gas-phase portion in the polymerization vessel. If such a deposit is solidified and mixed into the polymerization solution, a foreign matter is formed and thus sometimes causes failure of stable operation.

Conventionally, in production of an ethylene-vinyl alcohol copolymer, a stirring power per unit volume, which is an indicator of agitation strength, has been controlled. Such a stirring power is, however, influenced by various factors, such as the volume, the viscosity, and the density of a reaction liquid, the shape of a polymerization vessel, and the shape and the rotation number of a stirring blade. Accordingly, it is difficult to highly control the blocking properties of the ethylene units only by controlling the stirring power, resultantly leading to extension of ethylene chains (blocking of ethylene units), and the block character of the ethylene units in the ethylene-vinyl alcohol copolymer thus obtained is less than 0.90. Furthermore, the extension of ethylene chains causes stronger intermolecular hydrophobic interaction, and thus the rapid coatability, the initial adhesiveness, and the film strength used to be insufficient. As a result of intensive studies on this issue, the present inventors have found that a copolymer having shorter ethylene chains (ethylene units positioned at random) than those in the past is obtained by causing polymerization reaction to proceed under specific conditions and successfully improved these aspects of performance.

In the light of more highly controlling the block character of the ethylene units, the above production method preferably comprises: the polymerization step using the polymerization vessel connected to the heat exchanger via a pipe; and the steps of, in production of the ethylene-vinyl ester copolymer, (b) introducing the ethylene-containing gas in the gas-phase portion of the polymerization vessel into the heat exchanger, (c) feeding the vinyl ester to the heat exchanger, (d) contacting the vinyl ester with the ethylene-containing gas in the heat exchanger, and (e) discharging the vinyl ester containing ethylene dissolved therein from the heat exchanger and introducing the vinyl ester into the polymerization vessel. The vinyl ester may be directly fed to the polymerization vessel not through the heat exchanger. However, as the method described above, ethylene is efficiently absorbed by the vinyl ester by causing the vinyl ester to absorb ethylene in the heat exchanger in advance and then to be fed to the polymerization vessel, thereby the block character of the ethylene units can be highly controlled. Although part of the vinyl ester to be fed to the polymerization vessel may be contacted with the ethylene-containing gas in the heat exchanger, the total amount of the vinyl ester to be fed is preferably contacted with the ethylene-containing gas in the heat exchanger.

Although the heat exchanger to be used is not particularly limited, a heat exchanger with a large surface area is preferred in the light of efficient absorption of ethylene. Examples include a vertical wetted-wall heat exchanger, a vertical wetted-wall multitubular heat exchanger, a heat exchanger equipped with a jacket and/or a coil in a packed column type, a porous plate type, or a bubble cap type absorber. Among these, a vertical wetted-wall multitubular heat exchanger is more preferred.

In the apparatus illustrated in FIG. 1, a vertical wetted-wall multitubular heat exchanger is used as the heat exchanger 2. To the heat exchanger 2, a vinyl ester inlet pipe 10 is connected to feed the vinyl ester to an upper portion of the heat exchanger 2 through this pipe. The starting vinyl ester may be a vinyl ester alone or a mixed solution of the organic solvent and a vinyl ester, and the latter is preferred.

To the heat exchanger 2 illustrated in FIG. 1, refrigerant pipes 11 and 12 are connected. The positions of the pipes are not limited to those illustrated in FIG. 1, and it is preferred to feed a refrigerant from the refrigerant pipe 12 connected to a lower portion of the heat exchanger 2 and discharge the refrigerant from the refrigerant pipe 11 connected to the upper portion of the heat exchanger 2. With such connections, it is possible to efficiently cool the vinyl ester, resulting in high ethylene absorption efficiency. Examples of the refrigerant include, but not particularly limited to: aqueous solutions of an alcohol, such as methanol, ethanol, ethylene glycol, and glycerol; aqueous solutions of sodium chloride and calcium chloride; Freons; and the like. For the reasons of handleability, costs, and the like, aqueous solutions of an alcohol, particularly aqueous solutions of methanol are preferably used.

A gas discharge pipe 13 for discharging gas from the heat exchanger 2 is connected to the upper portion of the heat exchanger 2. A mist separator (not shown) may be connected to the gas discharge pipe 13. Droplets in the discharged gas are removed by the mist separator to allow mist-free ethylene to be collected or released. Such a mist separator is an apparatus for separating droplets suspended in gas using external force, such as gravity, centrifugal force, and electrostatic force, or a shielding or sieving effect. Examples of the mist separator include a gravity settler, a cyclone, an electrostatic precipitator, a scrubber, a bag filter, and a packed bed. Among these, a cyclone is preferred.

The method of contacting the vinyl ester with the ethylene-containing gas in the heat exchanger 2 is not particularly limited. Examples of the method include: a method comprising flowing the vinyl ester down from the upper portion of the heat exchanger 2 while feeding a pressurized ethylene-containing gas to the lower portion of the heat exchanger 2, and thus bringing them into countercurrent contact in the heat exchanger 2; a method of flowing the vinyl ester down from the upper portion of the heat exchanger 2 while feeding a pressurized ethylene-containing gas to the upper portion of the heat exchanger 2, and thus bringing them into cocurrent contact in the heat exchanger 2; and the like. In the light of efficient ethylene absorption, the former is preferred.

In the apparatus shown in FIG. 1, the two pipes 3 and 4 connect the polymerization vessel 1 to the heat exchanger 2. The ethylene-containing gas is introduced from the polymerization vessel 1 through the pipe 3 to the lower portion of the heat exchanger 2, and the vinyl ester having absorbed ethylene is introduced from the lower portion of the heat exchanger 2 through the pipe 4 to the polymerization vessel 1.

The vinyl ester is fed through the inlet pipe 10 to the heat exchanger 2. The vinyl ester introduced into the upper portion of the heat exchanger 2 absorbs ethylene while passing through the heat exchanger 2.

The ethylene-containing gas is introduced through the pipe 3 connected to the lower portion of the heat exchanger 2 into the heat exchanger 2. The pipe 3 on the heat exchanger side is connected to the lower portion of the heat exchanger 2, while the vinyl ester inlet pipe 10 is connected to the upper portion of the heat exchanger 2. The ethylene-containing gas flows up in the heat exchanger 2 while being brought into countercurrent contact with the vinyl ester.

As a result, ethylene in the gas is dissolved in the vinyl ester. The vinyl ester having absorbed ethylene is introduced through the pipe 4 into the polymerization vessel 1. For continuous production, ethylene is circulated in the polymerization vessel 1, the heat exchanger 2, and the pipes 3 and 4. Part of ethylene is contained in the vinyl ester to be discharged from the reaction liquid outlet pipe 9. Therefore, ethylene is replenished via at least one of the pipes 5, 6, and 7 from an ethylene supply source connected to the polymerization vessel 1.

Saponification Step

The ethylene-vinyl alcohol copolymer (A) is produced by saponifying the ethylene-vinyl ester copolymer obtained in the polymerization step. In this procedure, the ethylene-vinyl ester copolymer is preferably saponified by alcoholysis or hydrolysis reaction in the presence of a catalyst in an organic solvent. Examples of the catalyst used in the saponification step include: basic catalysts, such as sodium hydroxide, potassium hydroxide, and sodium methoxide; and acidic catalysts, such as sulfuric acid, hydrochloric acid, and p-toluenesulfonic acid. Examples of the organic solvent used in the saponification step include, but not particularly limited to, alcohols, such as methanol and ethanol; esters, such as methyl acetate and ethyl acetate; ketones, such as acetone and methyl ethyl ketone; and aromatic hydrocarbons, such as benzene and toluene. They may be used alone or in combination of two or more. Among these, it is convenient and preferred to perform saponification reaction in the presence of sodium hydroxide as a basic catalyst using methanol or a mixed solution of methanol and methyl acetate as a solvent. The amount of the saponification catalyst is preferably from 0.001 to 0.5 as a molar ratio to the vinyl ester units in the ethylene-vinyl ester copolymer. The molar ratio is more preferably 0.002 or more. The molar ratio is more preferably 0.4 or less and even more preferably 0.3 or less.

After the saponification step, a pulverization step and a drying step may be conducted. The pulverization step may be divided into a preliminary pulverization step and a main pulverization step. After the saponification step, if necessary, a washing step may be further conducted to remove impurities, such as sodium acetate.

Compound having Conjugated Double Bond

The water-based adhesive of the present invention preferably further comprises a compound with a molecular weight of 1000 or less having a conjugated double bond. The content of the compound is more preferably from 0.000001 to 0.01 parts by mass based on 100 parts by mass of the ethylene-vinyl alcohol copolymer (A). Use of the compound further improves the rapid coatability, the initial adhesiveness, and the film strength. Although this mechanism is not clearly understood, it is assumed to be because conjugated double bond moieties interact with the ethylene units in the ethylene-vinyl alcohol copolymer (A) in the polar solvent, thereby moderately inhibiting intermolecular interaction of the ethylene-vinyl alcohol copolymer (A) with each other to produce the water-based adhesive excellent in stability. The compound is preferably contained as a dispersant for the aqueous emulsion described later.

In the present invention, the compound with a molecular weight of 1000 or less having a conjugated double bond is preferably a compound having a conjugated double bond of aliphatic double bonds or a compound having a conjugated double bond of an aliphatic double bond and an aromatic ring. In the light of better polymerization stability during preparation of the aqueous emulsion, the former is more preferred. The molecular weight is 1000 or less, preferably 800 or less, and more preferably 500 or less.

The compound having a conjugated double bond of aliphatic double bonds is a compound having a conjugated double bond with a structure where carbon-carbon double bonds and carbon-carbon single bonds are alternately connected, wherein the number of the carbon-carbon double bonds is two or more. Specific examples include conjugated diene compounds with a conjugated structure where two carbon-carbon double bonds and one carbon-carbon single bond are alternately connected; conjugated triene compounds with a conjugated structure where three carbon-carbon double bonds and two carbon-carbon single bonds are alternately connected (e.g., 2,4,6-octatriene); conjugated polyene compounds with a conjugated structure where a more number of carbon-carbon double bonds and carbon-carbon single bonds are alternately connected; and the like. Among all, in the light of better polymerization stability during preparation of the aqueous emulsion, conjugated diene compounds are preferred. The compound with a molecular weight of 1000 or less having a conjugated double bond used in the present invention may contain independently a plurality of conjugated double bonds in one molecule; for example, compounds having three conjugated trienes in the same molecule, such as tung oil, may be included.

The compound with a molecular weight of 1000 or less having a conjugated double bond may have a functional group other than the conjugated double bond. Examples of such another functional group include: polar groups, such as carboxy groups and salts thereof, hydroxy groups, ester groups, carbonyl groups, ether groups, amino groups, dialkylamino groups, imino groups, amide groups, cyano groups, diazo groups, nitro groups, mercapto groups, sulfone groups, sulfoxide groups, sulfide groups, thiol groups, sulfonic acid groups and salts thereof, phosphoric acid groups and salts thereof, and a halogen atom; and nonpolar groups, such as phenyl groups. In the light of better polymerization stability during preparation of the aqueous emulsion, as such another functional group, polar groups are preferred, and carboxy groups and salts thereof and hydroxy groups are more preferred. Such another functional group may be directly bonded to a carbon atom in the conjugated double bond or bonded at a position away from the conjugated double bond. A multiple bond in such another functional group may be at a position conjugatable with the conjugated double bond; for example, 1-phenyl-1,3-butadiene having a phenyl group, sorbic acid having a carboxy group, and the like may be used as the compound having a conjugated double bond. The compound with a molecular weight of 1000 or less having a conjugated double bond may have an unconjugated double bond or an unconjugated triple bond.

Specific examples of the compound with a molecular weight of 1000 or less having a conjugated double bond include compounds having a conjugated double bond of aliphatic double bonds, such as 2,3-dimethyl-1,3-butadiene, 4-methyl-1,3-pentadiene, 1-phenyl-1,3-butadiene, sorbic acid, and myrcene; and compounds having a conjugated double bond of an aliphatic double bond and an aromatic ring, such as 2,4-diphenyl-4-methyl-1-pentene, α-methylstyrene polymers, and 1,3-diphenyl-1-butene.

The content of the compound with a molecular weight of 1000 or less having a conjugated double bond in the water-based adhesive is preferably 0.000001 to 0.01 parts by mass based on 100 parts by mass of the ethylene-vinyl alcohol copolymer (A). The content is more preferably 0.000002 parts by mass or more, and even more preferably 0.000003 parts by mass or more. The content is more preferably 0.0075 parts by mass or less, even more preferably 0.005 parts by mass or less, and particularly suitably 0.0025 parts by mass or less.

In the present invention, a method of adding the compound with a molecular weight of 1000 or less having a conjugated double bond is not particularly limited. Examples include: 1) a method comprising adding the compound to the ethylene-vinyl ester copolymer, followed by saponification; 2) a method comprising adding the compound during saponification of the ethylene-vinyl ester copolymer; 3) a method of spraying a liquid containing the compound on the ethylene-vinyl alcohol copolymer (A); 4) a method comprising impregnating the ethylene-vinyl alcohol copolymer (A) with a liquid containing the compound, followed by drying; 5) a method comprising preparing an aqueous solution containing the ethylene-vinyl alcohol copolymer (A) and the compound, followed by drying; 6) a method comprising preparing an aqueous solution containing the ethylene-vinyl alcohol copolymer (A) and the compound and using the aqueous solution as an aqueous dispersant solution for emulsion polymerization; and the like. Among these, the methods 2) and 6) are preferred in the light of the ease of adjusting the content of the compound.

Water-Based Adhesive

Using the ethylene-vinyl alcohol copolymer (A) thus obtained, the water-based adhesive for paper straws of the present invention is produced. The adhesive is a water-based adhesive comprising: the ethylene-vinyl alcohol copolymer (A); and water. In the water-based adhesive, the ethylene-vinyl alcohol copolymer (A) is preferably dissolved in water. It is another preferred embodiment that the ethylene-vinyl alcohol copolymer (A) is contained as a dispersant in the aqueous emulsion described later.

The water-based adhesive may contain a water-soluble organic solvent (alcohols, ketones, etc.) soluble in water at an arbitrary ratio. The content of the water-soluble organic solvent in the water-based adhesive is generally 50 parts by mass or less based on 100 parts by mass of the total of water and the water-soluble organic solvent and preferably 10 parts by mass or less, and the water-soluble organic solvent is more preferably not substantially contained in the water-based adhesive.

Examples of the method of dissolving the ethylene-vinyl alcohol copolymer (A) in water include a method comprising heating a mixture of the ethylene-vinyl alcohol copolymer (A) and water. The heating temperature is preferably 80° C. or more. The water-based adhesive as the aqueous solution of the ethylene-vinyl alcohol copolymer (A) thus obtained can be conveniently produced and excellent in costs.

The water-based adhesive is more preferably an aqueous emulsion containing a polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit. The adhesive as such an aqueous emulsion has even more excellent initial adhesiveness and dynamic water resistance.

Examples of the unsaturated ethylenic monomer unit contained in the polymer (B) include units derived from:

vinyl ester monomers, such as vinyl formate, vinyl acetate, vinyl propionate, and vinyl versatate;

(meth)acrylic acid monomers, such as acrylic acid and methacrylic acid;

(meth)acrylic ester monomers, such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl acrylate and quaternary compounds thereof, and dimethylaminoethyl methacrylate and quaternary compounds thereof;

styrene monomers, such as styrene, α-methylstyrene, p-styrenesulfonic acid, and sodium salts and potassium salts thereof;

diene monomers, such as butadiene, isoprene, and chloroprene;

olefin monomers, such as ethylene, propylene, and isobutylene;

acrylamide monomers, such as acrylamide, methacrylamide, N-methylolacrylamide, N,N-dimethylacrylamide, and acrylamide-2-methylpropanesulfonic acid and sodium salts thereof;

halogenated olefins, such as vinyl chloride, vinyl fluoride, vinylidene chloride, and vinylidene fluoride;

N-vinylpyrrolidone; and the like. Among all, the polymer (B) is preferably a polymer containing monomer units derived from at least one selected from the group consisting of a vinyl ester monomer, a (meth)acrylic ester monomer, a styrene monomer, and a diene monomer. The total content of the vinyl ester monomer, the (meth)acrylic ester monomer, the styrene monomer, and the diene monomer is preferably 70 mass % or more and more preferably 75 mass % or more based on the total monomer units of the polymer (B). Among all, the polymer (B) particularly preferably contains 75 mass % or more of the vinyl ester monomer units based on the total monomer units.

The dispersant contained in the aqueous emulsion is not particularly limited, and a known dispersant for emulsion polymerization of an unsaturated ethylenic monomer may be used. Among all, in the light of further improved initial adhesiveness and dynamic water resistance of the resulting adhesive, the dispersant is preferably the ethylene-vinyl alcohol copolymer (A) or a PVA (C) described later and is more preferably the ethylene-vinyl alcohol copolymer (A).

As a method of producing the aqueous emulsion, a method comprising emulsion polymerizing the unsaturated ethylenic monomer using a polymerization initiator in the presence of the dispersant is preferred.

In the above method, when the dispersant is charged into the polymerization vessel, methods of charging and adding the dispersant are not particularly limited. Examples include a method comprising initially adding the dispersant in bulk into the polymerization vessel and a method comprising continuously adding the dispersant during polymerization. Among all, in the light of increasing a ratio of grafting the dispersant to the unsaturated ethylenic monomer, the method comprising initially adding the dispersant in bulk into the polymerization vessel is preferred. In this situation, a preferred method comprises: adding the dispersant to cold water or warm water heated in advance; and heating and stirring at a temperature from 80° C. to 90° C. for homogeneous dispersion of the dispersant. In the case of using the ethylene-vinyl alcohol copolymer (A) or the PVA (C) as a dispersant, it is preferred to heat water containing the ethylene-vinyl alcohol copolymer (A) or the PVA (C) for dissolution before initiating the emulsion polymerization, followed by cooling and purging it with nitrogen. The heating temperature is preferably 80° C. or more.

In the emulsion polymerization, the amount of the dispersant is preferably from 0.2 to 80 parts by mass based on 100 parts by mass of the unsaturated ethylenic monomer. When the amount of the dispersant is the lower limit or more, aggregation of dispersoid particles is less likely to be formed in the aqueous emulsion and thus the polymerization stability tends to be excellent during preparation of the aqueous emulsion. The amount of the dispersant is more preferably 0.5 parts by mass or more, even more preferably 1 part by mass or more, particularly preferably 2 parts by mass or more, and most preferably 4 parts by mass or more. When the amount of the dispersant is the upper limit or less, the viscosity of the polymerization liquid is not too high, leading to a tendency of causing the polymerization to proceed homogeneously and efficiently removing the heat of polymerization. The amount of the dispersant is more preferably 60 parts by mass or less, even more preferably 50 parts by mass or less, and particularly preferably 40 parts by mass or less.

As the polymerization initiator in the emulsion polymerization, it is possible to use water-soluble single initiators or water-soluble redox initiators generally used for emulsion polymerization. One type of these initiators may be used singly or two or more types of them may be used together. Among all, redox initiators are preferred.

Examples of the water-soluble single initiators include: azo initiators; peroxides, such as hydrogen peroxide and persulfates (potassium, sodium, and ammonium salts); and the like. Examples of the azo initiators include 2,2′-azobis(isobutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), and the like.

As the redox initiator, it is possible to use those obtained by combining an oxidant with a reducing agent. Such an oxidant is preferably a peroxide. Examples of the reducing agent include metal ions, reducing compounds, and the like. Examples of the combinations of the oxidant with the reducing agent include combination of a peroxide with a metal ion, combination of a peroxide with a reducing compound, and combination of a peroxide with a metal ion and a reducing compound. Examples of the peroxide include: hydroxyperoxides, such as hydrogen peroxide, cumene hydroxyperoxide, and t-butyl hydroxyperoxide; persulfates (potassium, sodium, and ammonium salts); t-butyl peracetate; peracid esters (t-butyl perbenzoate); and the like. Examples of the metal ion include metal ions, such as Fe²⁺, Cr²⁺, V²⁺, Co²⁺, Ti³⁺, and Cu⁺, capable of undergoing one-electron transfer. Examples of the reducing compound include sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, fructose, dextrose, sorbose, inositol, Rongalite, and ascorbic acid. Among them, combination of one or more oxidants selected from the group consisting of hydrogen peroxide, potassium persulfate, sodium persulfate, and ammonium persulfate with one or more reducing agents selected from the group consisting of sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, Rongalite, and ascorbic acid is preferred, and combination of hydrogen peroxide with one or more reducing agents selected from the group consisting of sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, Rongalite, and ascorbic acid is more preferred.

In the emulsion polymerization, alkali metal compounds, surfactants, buffers, polymerization regulators, and the like may be appropriately used as long as the effects of the present invention are not impaired.

Examples of the alkali metal compounds include, but not particularly limited to, compounds containing sodium, potassium, rubidium, cesium, and the like. The alkali metal compounds may be alkali metal ions themselves or compounds containing alkali metal.

In the case of using the alkali metal compounds, the content (in terms of alkali metal) may be appropriately selected in accordance with the type of alkali metal compound and is preferably from 100 to 15000 ppm based on the total solid content of the water-based adhesive, more preferably from 120 to 12000 ppm, and even more preferably from 150 to 8000 ppm. When the alkali metal compound content is 100 ppm or more, the emulsion polymerization tends to have excellent stability during preparation of the aqueous emulsion. When the content is 15000 ppm or less, a film formed from the aqueous emulsion is less likely to be colored. It should be noted that the alkali metal compound content may be measured with an ICP emission spectrometer. The term “ppm” herein means “mass ppm”.

Specific examples of the compounds containing alkali metal include weak base alkali metal salts (e.g., alkali metal carbonate, alkali metal acetate, alkali metal bicarbonate, alkali metal phosphate, alkali metal sulfate, alkali metal halide salt, alkali metal nitrate), strong base alkali metal compounds (e.g., hydroxide of alkali metal, alkoxide of alkali metal), and the like. One type of these alkali metal compounds may be used singly or two or more types of them may be used together.

Examples of the weak base alkali metal salts include alkali metal carbonates (e.g., sodium carbonate, potassium carbonate, rubidium carbonate, cesium carbonate), alkali metal bicarbonates (e.g., sodium hydrogen carbonate, potassium hydrogen carbonate, etc.), alkali metal phosphates (sodium phosphate, potassium phosphate, etc.), alkali metal carboxylates (sodium acetate, potassium acetate, cesium acetate, etc.), alkali metal sulfates (sodium sulfate, potassium sulfate, cesium sulfate, etc.), alkali metal halide salts (cesium chloride, cesium iodide, potassium chloride, sodium chloride, etc.), and alkali metal nitrates (sodium nitrate, potassium nitrate, cesium nitrate, etc.). Among them, in the light of giving basicity to the aqueous emulsion, alkali metal carboxylates, alkali metal carbonates, and alkali metal bicarbonates, which are capable of behaving as weak acid-strong base salt, are preferred and alkali metal carboxylates are more preferred.

Use of such a weak base alkali metal salt causes the weak base alkali metal salt to act as a pH buffer in the emulsion polymerization and thus allows the emulsion polymerization to stably proceed.

As such a surfactant, any of nonionic surfactants, anionic surfactants, and cationic surfactants may be used. Examples of the nonionic surfactant include, but not particularly limited to, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene fatty acid ester, polyoxyalkylene alkyl ether, polyoxyethylene derivatives, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene sorbitol fatty acid ester, glycerol fatty acid ester, and the like. Examples of the anionic surfactant include, but not particularly limited to, alkyl sulfate, alkylaryl sulfate, alkyl sulphonate, sulfate of hydroxyalkanol, sulfosuccinic acid ester, sulfate and phosphate of alkyl or alkylaryl polyethoxyalkanol, and the like. Examples of the cationic surfactant include, but not particularly limited to, alkylamine salt, quaternary ammonium salt, polyoxyethylene alkylamine, and the like. The amount of the surfactant is preferably 2 parts by mass or less based on 100 parts by mass of the unsaturated ethylenic monomer (e.g., vinyl acetate). The amount of the surfactant of more than 2 parts by mass is not preferred because of sometimes causing decrease in water resistance, hot water resistance, and boiling resistance.

Examples of the buffer include: acids, such as acetic acid, hydrochloric acid, and sulfuric acid; bases, such as ammonia, amine, caustic soda, caustic potash, and calcium hydroxide; alkali carbonate; phosphate; acetate; and the like. Examples of the polymerization regulator include mercaptans, alcohols, and the like.

The temperature of the emulsion polymerization is, but not particularly limited to, preferably approximately from 20° C. to 85° C. and more preferably approximately from 40° C. to 85° C.

When the ethylene-vinyl alcohol copolymer (A) is used as the dispersant, it is possible to use the resulting aqueous emulsion as a water-based adhesive. The aqueous emulsion comprising: the ethylene-vinyl alcohol copolymer (A) as a dispersant; and a polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit has even more excellent initial adhesiveness and dynamic water resistance.

It is preferred that, to the aqueous emulsion after emulsion polymerization, an aqueous solution of the ethylene-vinyl alcohol copolymer (A) is further added. As the aqueous solution of the ethylene-vinyl alcohol copolymer (A), those described above are used. If the aqueous emulsion is obtained using a dispersant other than the ethylene-vinyl alcohol copolymer (A), it is possible to obtain a water-based adhesive containing the ethylene-vinyl alcohol copolymer (A) by adding the aqueous solution of the ethylene-vinyl alcohol copolymer (A) to the aqueous emulsion. The water-based adhesive of the aqueous emulsion thus obtained in which the ethylene-vinyl alcohol copolymer (A) is dissolved has even more excellent dynamic water resistance and also causes the film to be formed to have further improved strength. In particular, it is preferred that, to the aqueous emulsion comprising: the ethylene-vinyl alcohol copolymer (A) as a dispersant; and a polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit, the aqueous solution of the ethylene-vinyl alcohol copolymer (A) is further added. The aqueous emulsion thus obtained, comprising: dispersed particles containing the ethylene-vinyl alcohol copolymer (A) as a dispersant and the polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit; and the ethylene-vinyl alcohol copolymer (A) dissolved therein, exhibits particularly excellent performance as a water-based adhesive.

The content of the ethylene-vinyl alcohol copolymer (A) in the water-based adhesive of the present invention is preferably 0.1 mass % or more. If the content is less than 0.1 mass %, there is a risk of decreasing any of the rapid coatability, the film strength, the initial adhesiveness, the dynamic water resistance, and the acid resistance. The content of the ethylene-vinyl alcohol copolymer (A) is more preferably 1 mass % or more, even more preferably 3 mass % or more, particularly preferably 5 mass % or more, and most preferably 6 mass % or more. Meanwhile, the content of the ethylene-vinyl alcohol copolymer (A) is preferably 50 mass % or less. If the content is more than 50 mass %, there is a risk of excessively increasing the viscosity of the water-based adhesive and causing difficulty in handling. The content is more preferably 35 mass % or less, even more preferably 25 mass % or less, particularly preferably 18 mass % or less, and most preferably 13 mass % or less.

In the emulsion, a mass ratio (A)/(B) of the ethylene-vinyl alcohol copolymer (A) to the polymer (B) is preferably from 2/98 to 80/20. When the mass ratio (A)/(B) is 2/98 or more, the film strength is further improved and straws obtained from the film have further improved strength. From this aspect, the mass ratio (A)/(B) is more preferably 5/95 or more, even more preferably 8/92 or more, particularly preferably 10/90 or more, and most preferably 12/88 or more. In the light of obtaining straws having extremely high strength, the mass ratio is preferably 15/85 or more and more preferably 17/83 or more. Meanwhile, when the mass ratio (A)/(B) is 80/20 or less, the dynamic water resistance is further improved. From this aspect, the mass ratio (A)/(B) is more preferably 70/30 or less, even more preferably 60/40 or less, and particularly preferably 50/50 or less. In the light of obtaining straws having extremely high dynamic water resistance, the mass ratio (A)/(B) is preferably 40/60 or less and more preferably 30/70 or less.

The water-based adhesive of the present invention may further contain the PVA (C) other than the ethylene-vinyl alcohol copolymer (A). Use of the ethylene-vinyl alcohol copolymer (A) together with the PVA (C) allows cost containment while maintaining the performance. In the water-based adhesive of the present invention, the PVA (C) is preferably dissolved in water. It is another preferred embodiment that the PVA (C) is contained as a dispersant of the aqueous emulsion. The water-based adhesive containing the PVA (C) can be obtained by: a method comprising heating a mixture containing the ethylene-vinyl alcohol copolymer (A), the PVA (C), and water to dissolve them; a method comprising mixing an aqueous solution of the ethylene-vinyl alcohol copolymer (A) with an aqueous solution of the PVA (C); a method comprising mixing the aqueous emulsion containing the ethylene-vinyl alcohol copolymer (A) as a dispersant and the polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit with the aqueous solution of the PVA (C); a method comprising mixing the aqueous emulsion containing the PVA (C) as a dispersant and the polymer (B), as a dispersoid, containing an unsaturated ethylenic monomer unit with the aqueous solution of the ethylene-vinyl alcohol copolymer (A); and the like. The aqueous solution of the PVA (C) can be obtained by the same method as that for the aqueous solution of the ethylene-vinyl alcohol copolymer (A).

The polyvinyl alcohol (C) preferably has a viscosity-average degree of polymerization from 200 to 5000. The PVA (C) preferably has a degree of saponification from 80 to 99.7 mol %. The viscosity-average degree of polymerization and the degree of saponification of the PVA (C) can be determined in accordance with JIS K6726(1994).

A method of producing a PVA (C) is not particularly limited. Examples include a known method in which a vinyl ester is polymerized to obtain a polyvinyl ester and saponify it. Examples of the vinyl ester to be used for production of the PVA (C) include those described above to be used for production of the ethylene-vinyl ester copolymer.

The PVA (C) may contain monomer units other than the vinyl alcohol units and the vinyl ester units as long as the effects of the present invention are not impaired. Examples of such other monomer units include those described above as the monomer units other than the vinyl alcohol units, the ethylene units, and the vinyl ester units optionally contained in the ethylene-vinyl alcohol copolymer (A). The content of the monomer units other than the vinyl alcohol units and the vinyl ester units is, depending on the intended objective, use, and the like, preferably 10 mol % or less, more preferably less than 5 mol %, even more preferably less than 1 mol %, and particularly preferably less than 0.5 mol %.

When the water-based adhesive contains the PVA (C), a mass ratio (C)/(A) of the PVA to the ethylene-vinyl alcohol copolymer (A) is preferably from 1/99 to 50/50. The mass ratio (C)/(A) in such a range allows cost reduction while maintaining the performance of the water-based adhesive. The mass ratio (C)/(A) is more preferably 40/60 or less, even more preferably 30/70 or less, and particularly preferably 25/75 or less.

The water-based adhesive preferably further contains an inorganic filler. This further improves the initial adhesiveness. Examples of the inorganic particles include calcium carbonate, clay, kaolin, talc, titanium oxide, and the like. In the water-based adhesive, a mass ratio [Inorganic Filler/Copolymer (A)] of the inorganic filler to the ethylene-vinyl alcohol copolymer (A) is preferably from 1/99 to 99.5/0.5. The mass ratio [Inorganic Filler/Copolymer (A)] is more preferably 10/90 or more and even more preferably 30/70 or more. Meanwhile, the mass ratio [Inorganic Filler/Copolymer (A)] is more preferably 80/20 or less and even more preferably 75/25 or less.

The water-based adhesive of the present invention preferably has a solid content of 10 mass % or more, more preferably 20 mass % or more, and even more preferably 25 mass % or more. Meanwhile, the solid content is preferably 60 mass % or less, more preferably 45 mass % or less, and even more preferably 40 mass % or less. When the solid content is 10 mass % or more, the water-based adhesive tends to have excellent viscosity stability. Meanwhile, when the solid content is 60 mass % or less, the aqueous emulsion tends to provide long open time and excellent handleability.

The water-based adhesive may contain various additives as long as the effects of the present invention are not impaired. Examples of such an additive include organic solvents (aromatic compounds, such as toluene and xylene, alcohols, ketones, esters, halogen-containing solvents, etc.), crosslinking agents, plasticizers, suspending agents, thickeners, fluidity modifiers, preservatives, defoamers, organic fillers, wetting agents, colorants, binders, water retention agents, polyethylene oxide, mildewcides, deodorants, perfumes, and the like.

Examples of the additive also include: inorganic dispersants, including metal salts of phosphoric acid compounds, such as sodium polyphosphate and sodium hexametaphosphate, water glasses, and the like; polyacrylic acid and salts thereof; sodium alginate; anionic polymer compounds, including α-olefin-maleic anhydride copolymers, and metal salts thereof; surfactants, including nonionic surfactants, such as ethylene oxide adducts of higher alcohols and copolymers of ethylene oxide and propylene oxide. Addition of them improves the fluidity of the adhesive. To further improve the dynamic water resistance, it is also possible to contain one or more crosslinking agents selected from water-soluble metal compounds, colloidal inorganics, polyamideamine-epichlorohydrin adducts, and glyoxal-based resins. In this situation, examples of the water-soluble metal compound include aluminium chloride, aluminium nitrate, ammonium zirconium carbonate, titanium lactate, and the like. Examples of the colloidal inorganic include colloidal silica, alumina sol, and the like. Examples of the polyamideamine-epichlorohydrin adduct include addition products of epichlorohydrin to various polyamideamines. Examples of the glyoxal-based resin include urea-glyoxal resins and the like. Use of the water-soluble metal salt or the colloidal inorganic together with the polyamideamine-epichlorohydrin adduct or the glyoxal-based resin more improves the dynamic water resistance. It is also possible to use, as long as not impairing the performance, methylol group-containing compounds (resins), epoxy compounds (resins), aziridine group-containing compounds (resins), oxazoline group-containing compounds (resins), carbodiimide compounds, aldehyde compounds (resins), and the like together with the above crosslinking agents. To further improve the adhesive strength, it is also possible to add: boric acid; borax; water-soluble boron compounds, including boric acid esters of polyalcohols, such as glycerol and ethylene glycol, and the like; sodium naphthalenesulfonate-formalin condensates; and the like. It is further possible to add, as other additives: natural sizing agents, such as starch, casein, gelatin, guar gum, gum arabic, and sodium alginates; modified natural sizing agents, such as carboxymethylcellulose, oxidized starch, and methylcellulose; and the like. One type of them may be used singly or two or more types of them may be used together.

The total of the additives other than the ethylene-vinyl alcohol copolymer (A), the polymer (B), the PVA (C), the compound with a molecular weight of 1000 or less having a conjugated double bond, water, the water-soluble organic solvent, the alkali metal compound, and the inorganic filler in the water-based adhesive is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 25 parts by mass or less, and particularly preferably 10 mass parts by or less based on 100 parts by mass of the ethylene-vinyl alcohol copolymer (A).

A paper straw obtained by adhering a paper substrate to each other using the water-based adhesive is a preferred embodiment of the present invention. The paper straw is excellent in the strength, the dynamic water resistance, and the acid resistance. In addition, use of the adhesive of the present invention excellent in the initial adhesiveness and the rapid coatability allows efficient production of such a paper straw.

It is possible to obtain the paper straw by a general production method, and for example, it is possible to produce it by a method comprising: applying the water-based adhesive on a paper substrate in a tape form; laminating and winding the paper substrate on a metal rod in a spiral; followed by drying, and the like.

A paper straw comprising the ethylene-vinyl alcohol copolymer (A) having a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99 is also preferably used. Being excellent in the film strength, the dynamic water resistance, and the acid resistance, the ethylene-vinyl alcohol copolymer (A) is also preferably used as a paper treatment agent used for producing a paper substrate for paper straws, and the like. The paper straw containing the ethylene-vinyl alcohol copolymer (A) is thus also excellent in the strength, the dynamic water resistance, and the acid resistance.

EXAMPLES

With reference to Examples, the present invention will then be described more specifically, but the present invention is not at all limited to these Examples and many modifications may be made within the technical spirit of the present invention by those with ordinary knowledge in the art. It should be noted that “%” and “part(s)” in Examples and Comparative Examples below respectively refer to “mass %” and “part(s) by mass” unless otherwise specified.

Viscosity-Average Degree of Polymerization and Degree of Saponification of Ethylene-Vinyl Alcohol Copolymer

The viscosity-average degree of polymerization and the degree of saponification of the ethylene-vinyl alcohol copolymer were determined in accordance with the method described in JIS K6726 (1994).

Block Character of Ethylene Units of Ethylene-Vinyl Alcohol Copolymer

The ethylene-vinyl alcohol copolymer was saponified to a degree of saponification of 99.9 mol % or more, followed by thorough washing with methanol and then drying at 90° C. under reduced pressure for 2 days. The copolymer thus obtained was dissolved in DMSO-d₆ and analyzed by 600 MHz ¹³C-NMR at 80° C. Using the molar fraction of vinyl alcohol-ethylene two-unit chains (AE), the molar fraction of the vinyl alcohol units (A), and the molar fraction of the ethylene units (E) assigned and calculated from the obtained spectrum as described in T. Moritani and H. Iwasaki, 11, 1251-1259, Macromolecules (1978), the block character (η) of the ethylene units was determined according to the following equation:

η=(AE)/{2×(A)×(E)}

The initial adhesiveness, the dynamic water resistance, the film hardness, the rapid coatability, and the acid resistance of the prepared water-based adhesive were evaluated by the following methods.

(1) Initial Adhesiveness

In an atmosphere at 20° C. and 65% RH, each water-based adhesive is applied on kraft paper with a No. 6 wire bar and adhered, followed by pressing three times with a hand roller. Then, the time was measured from the completion of pressing to, when the adhered kraft paper tried to be separated with hands, complete breaking of the kraft paper without being separated from each other. Evaluations were made in accordance with the following criteria.

A: the kraft paper was completely broken in less than 10 seconds. B: the kraft paper was completely broken in 10 seconds or more and less than 30 seconds. C: the kraft paper was completely broken in 30 seconds or more and less than 1 minute. D: the kraft paper was completely broken in 1 minute or more.

(2) Dynamic Water Resistance

In an atmosphere at 20° C. and 65% RH, each water-based adhesive was applied on kraft paper with a No. 6 wire bar and adhered, followed by pressing three times with a hand roller and curing for 24 hours. The laminated kraft paper thus obtained was immersed in water at 30° C. for 24 hours while slowly stirred with a stirrer, and then, remained in the wet condition, the ends of the kraft paper were peeled from each other to observe the state of adhesion and evaluated by the following criteria.

A: the kraft paper was completely broken. B: part of the kraft paper was broken. C: some resistance was felt in separating the kraft paper from each other. D: almost no resistance was felt in separating the kraft paper from each other.

(3) Film Hardness (Measurement of Young's Modulus)

Each film with a width of 10 mm was subjected to humidity control in an atmosphere at 20° C. and 65% RH for 1 week and then a tensile test (a length between the chucks of 20 mm, a tensile speed of 100 mm/min) using an Autograph, followed by evaluation of the film hardness by the following criteria.

A: the Young's modulus was 40 kgf/mm² or more. B: the Young's modulus was 30 kgf/mm² or more and less than 40 kgf/mm². C: the Young's modulus was 20 kgf/mm² or more and less than 30 kgf/mm². D: the Young's modulus was less than 20 kgf/mm².

(4) Rapid Coatability

Using a three-roll coater, the water-based adhesives obtained by Examples and Comparative Examples below were evaluated. FIG. 3 illustrates the three-roll coater used in this procedure. The temperature on the surface of each roll was adjusted at 30° C. A water-based adhesive [IV] thus prepared was supplied between a roll [II] and a roll [III] and the rolls were rotated to provide a surface speed of a roll [I] of 100 m/min, thereby observing and evaluating the jumping phenomena. Whether droplets of the water-based adhesive jumped out (jumping phenomena) between the rolls [I] and [II] was visually assessed by the following criteria.

A: no droplets were jumped out at all. B: a small number of droplets was jumped out. C: a large number of droplets was jumped out.

(5) Acid Resistance Test

In an atmosphere at 20° C. and 65% RH, each water-based adhesive was applied on kraft paper with a No. 6 wire bar and adhered, followed by pressing three times with a hand roller and curing for 24 hours. The laminated kraft paper thus obtained was immersed in grape juice (pH 3.5) at 20° C. for 24 hours while slowly stirred with a stirrer, and then, remained in the wet condition, the occurrence of delamination at the ends of the kraft paper were observed and evaluated by the following criteria.

A: delaminated B: not delaminated

Production of Copolymer (A-1)

FIG. 1 illustrates a schematic view of the polymerization apparatus used. Into an almost columnar polymerization vessel 1 [volume: 7 kl, vessel inner diameter D: 1.8 m] equipped with a Maxblend blade [from Kobelco Eco-Solutions Co., Ltd., stirring blade diameter (diameter) d: 1.1 m, blade (paddle) width b: 1.5 m] as a stirring blade 8, ethylene was introduced from a pipe 5 to have an inner ethylene pressure of 0.23 MPa and also a 1 mass % solution of 2,2′-azobis-(4-methoxy-2,4-dimethylvaleronitrile) as a polymerization initiator in methanol was introduced at a rate of 3 L/hr from a pipe 6. Furthermore, via an inlet pipe 10 and a heat exchanger 2, a vinyl acetate containing solution (vinyl acetate: 777 L/hr, methanol: 170 L/hr) was introduced into the polymerization vessel 1. The ethylene-containing gas was introduced from the polymerization vessel 1 through a pipe 3 into the heat exchanger 2. The vinyl acetate containing solution absorbed ethylene by flowing down along the pipe surface and poured into the polymerization vessel 1 via a pipe 4 to be mixed with the reaction liquid and subjected to continuous polymerization with ethylene. The polymerization liquid was continuously removed through a pipe 9 to keep the liquid surface in the polymerization vessel 1 at a constant level. The conditions were regulated to have the polymerization ratio of vinyl acetate at the outlet of the polymerization vessel 1 at 30%. In addition, the conditions were regulated to have the stirring power Pv per unit volume of 2.2 kW/m³ and the Froude number Fr of 0.13. The reaction liquid was stirred in the state where the entire blade (paddle) was immersed in the reaction liquid and also the surface level of the liquid was close to the upper end of the blade (paddle). The residence time of the reaction liquid in the polymerization vessel was 5 hours. The temperature at the outlet of the polymerization vessel was 60° C. Methanol vapor was introduced into the continuously removed polymerization liquid to remove the unreacted vinyl acetate monomer and thus to obtain a solution of an ethylene-vinyl acetate copolymer in methanol (concentration of 32%).

To the solution of an ethylene-vinyl acetate copolymer in methanol (concentration of 32%) obtained in the above polymerization step, a solution of sodium hydroxide as a saponification catalyst in methanol (concentration of 4 mass %) was added to have a molar ratio of sodium hydroxide to the vinyl acetate units in the ethylene-vinyl acetate copolymer of 0.012. Based on 100 parts by mass of the ethylene-vinyl acetate copolymer, 0.00009 parts by mass in terms of solid contents of a solution of sorbic acid in methanol (concentration of 10 mass %) was further added, and the mixture thus obtained was mixed with a static mixer and then placed on a belt to be retained at 40° C. for 18 minutes for proceeding of saponification reaction. Subsequently, the mixture was pulverized and dried to give an ethylene-vinyl alcohol copolymer (A-1) (hereinafter, referred to as a copolymer (A-1)). The copolymer (A-1) thus obtained had an ethylene unit content of 2 mol %, a viscosity-average degree of polymerization of 1700, a degree of saponification of 98.5 mol %, and a block character of the ethylene units of 0.95, and the amount of sorbic acid was 0.00018 parts by mass based on 100 parts by mass of the ethylene-vinyl alcohol copolymer.

Production of Copolymers (A-2), (A-3), (A-5), and (A-6) and PVA-7

Ethylene-vinyl alcohol copolymers (copolymers (A-2), (A-3), (A-5), and (A-6)) and a PVA (PVA-7) were produced in the same method as that in Example 1 except for changing: during polymerization, the feeding amounts and the polymerization ratios of ethylene, vinyl acetate, methanol, and the initiator, the type of the stirring blade, the stirring power Pv, the Froude number Fr; and during saponification, the concentration of the ethylene-vinyl ester copolymer solution, the molar ratio of sodium hydroxide, and the type and the amount of the compound containing a conjugated double bond. Table 1 collectively shows the polymerization conditions and the saponification conditions during production, the degrees of polymerization, the degrees of saponification, the ethylene units, and the block characters of the polymers thus obtained, and the types and the contents of the compounds containing a conjugated double bond.

Production of Copolymer (A-4)

Into a polymerization vessel 1 [volume: 7 kl, vessel inner diameter D: 1.8 m] equipped with a two-stage inclined paddle blade [stirring blade diameter (diameter) d: 1.5 m, blade (paddle) width b: 0.88 m] as the stirring blade 8, ethylene was introduced from the pipe 5 to have an inner ethylene pressure of 0.61 MPa and also a 1 mass % solution of 2,2′-azobis-(4-methoxy-2,4-dimethylvaleronitrile) as a polymerization initiator in methanol was introduced at a rate of 9 L/hr from the pipe 6. Furthermore, from an inlet pipe 7, a vinyl acetate containing solution (vinyl acetate: 751 L/hr, methanol: 190 L/hr) was introduced into the polymerization vessel 1. The polymerization liquid was continuously removed through the pipe 9 to keep the liquid surface in the polymerization vessel 1 at a constant level. The conditions were regulated to have the polymerization ratio at the outlet of the polymerization vessel 1 at 43%. In addition, the conditions were regulated to have the stirring power Pv of 2 kW/m³ and the Froude number Fr of 0.18. The residence time of the reaction liquid in the polymerization vessel was 5 hours. The temperature at the outlet of the polymerization vessel was 60° C. Methanol vapor was introduced into the continuously removed polymerization liquid to remove the unreacted vinyl acetate monomer and thus to obtain a solution of an ethylene-vinyl acetate copolymer (A-4) in methanol (concentration of 40%). The ethylene-vinyl alcohol copolymer (A-4) was produced by performing a saponification step in the same method as that in Example 1 except for changing the concentration of the ethylene-vinyl ester copolymer solution and the molar ratio of sodium hydroxide. Table 1 collectively shows the polymerization conditions and the saponification conditions during production, the degree of polymerization, the degree of saponification, the ethylene units, and the block character of the polymer thus obtained, and the type and the content of the compound containing a conjugated double bond.

Example 1

Preparation of Water-Based Adhesive

Powder (15 parts) of the ethylene-vinyl alcohol copolymer (A-1) thus obtained was charged into water (85 parts, 20° C.) under stirring and heated to 95° C. to dissolve the ethylene-vinyl alcohol copolymer, thereby obtaining a water-based adhesive. The initial adhesiveness, the dynamic water resistance, the film hardness, the rapid coatability, and the acid resistance of the water-based adhesive thus obtained were evaluated by the methods described above. The results are shown in Table 2.

Example 2

Powder (15 parts) of the ethylene-vinyl alcohol copolymer (A-1) and calcium carbonate (30 parts) were sufficiently dry blended and then charged into water (85 parts, 20° C.) under stirring and heated to 95° C. to dissolve the ethylene-vinyl alcohol copolymer, thereby obtaining a water-based adhesive. In the same manner as in Example 1, production and evaluation of the water-based adhesive were performed. The results are shown in Table 2.

Example 3

Production of Aqueous Emulsion Em-1

Into a 1-liter glass polymerization vessel equipped with a reflux condenser, a dropping funnel, a thermometer, and a nitrogen inlet port, 275 g of ion exchange water was charged and heated at 95° C. Then, 19.5 g of the PVA-7 was added and stirred for 45 minutes to be dissolved. Further, 0.3 g of sodium acetate was added and mixed to be dissolved. The aqueous solution of the PVA-7 dissolved therein was then cooled and purged with nitrogen, followed by raising the temperature to 60° C. while stirring at 200 rpm. Then, 2.4 g of a 20 mass % aqueous solution of tartaric acid and 3.2 g of a 5 mass % hydrogen peroxide solution were added in shots, and then 27 g of vinyl acetate was charged to initiate polymerization. Completion of the initial polymerization was confirmed (the amount of residual vinyl acetate was less than 1%) 30 minutes after initiating the polymerization. Then, 1 g of a 10 mass % aqueous solution of tartaric acid and 3.2 g of a 5 mass % hydrogen peroxide solution were added in shots, and then 251 g of vinyl acetate was continuously added for 2 hours. The polymerization temperature was maintained at 80° C. and the polymerization was completed to obtain an aqueous polyvinyl acetate-based emulsion (Em-1) with a solid content concentration of 50 mass %.

Preparation of Water-Based Adhesive

Powder (12 parts) of the ethylene-vinyl alcohol copolymer (A-2) thus obtained was charged into water (88 parts, 20° C.) under stirring and heated to 95° C. to dissolve the ethylene-vinyl alcohol copolymer, thereby obtaining an aqueous solution with a concentration of 12%. An aqueous solution (100 parts) of the copolymer (A-2) thus obtained and the aqueous emulsion Em-1 (107 parts) were blended at room temperature to obtain a water-based adhesive. The initial adhesiveness, the dynamic water resistance, the film hardness, the rapid coatability, and the acid resistance of the water-based adhesive thus obtained were evaluated by the methods described above. The results are shown in Table 2.

Examples 4 through 11 and Comparative Examples 1 through 4

Production of Aqueous Emulsions Em-2 through EM-8

As shown in Table 2, aqueous emulsions were prepared in the same manner as in Example 3 except for using predetermined amounts of copolymers (A-2) through (A-5) instead of the PVA-7 in Example 3 and changing the solid contents. It should be noted that a copolymer (A-6) was insoluble in water and it was not possible to obtain an aqueous emulsion.

Preparation of Water-Based Adhesives

As shown in Table 2, water-based adhesives were prepared in the same manner as in Example 3 except for using predetermined amounts of Em-1 through Em-8 and using predetermined amounts of aqueous solutions of the copolymers (A-1), (A-2), (A-4), and (A-5) and the PVA-7. The initial adhesiveness, the dynamic water resistance, the film hardness, the rapid coatability, and the acid resistance of the water-based adhesives thus obtained were evaluated by the methods described above. The results are shown in Table 2.

TABLE 1 Polymerization Conditions Vinyl Polymerization Supply of Ethylene Acetate Methanol Initiator Ratio Stirring Blade Pv Fr Vinyl Acetate Mpa L/hr L/hr L/hr % — kW/m³ — — A-1 0.23 777 170 3 30 Maxblend Blade 2.2 0.13 Heat Exchanger A-2 0.47 820 128 2.5 28 Maxblend Blade 3 0.11 Heat Exchanger A-3 0.69 682 185 82.6 70 Maxblend Blade 1.9 0.1 Heat Exchanger A-4 0.61 751 190 9 43 Two-Stage Inclined 2 0.18 Direct Paddle Blade A-5 0.1 745 2 202.7 78 Maxblend Blade 6.9 0.21 Heat Exchanger A-6 ³⁾ 1.77 735 174 41 50 Anchor Blade 1.8 0.17 Heat Exchanger PVA-7 0 700 280 2.5 35 Maxblend Blade 2.1 0.14 Heat Exchanger Ethylene-Vinyl Alcohol Copolymer Saponification Conditions Compound Containing PVAc NaOH Ethylene Degree of Conjugated Double Bond Concentration molar Units Degree of Saponification Block Content ²⁾ mass % ratio (mol %) Polymerization (mol %) Character Type ¹⁾ (parts by mass) A-1 32 0.012 2 1700 98.5 0.95 SA 1.8 × 10⁻⁴ A-2 30 0.012 4 1700 98.5 0.97 SA 1.8 × 10⁻⁴ A-3 45 0.015 10 400 98 0.98 — — A-4 40 0.02 6 1000 99.2 0.85 SA 1.8 × 10⁻⁴ A-5 45 0.008 2 500 88 1.01 SA 1.8 × 10⁻⁴ A-6 ³⁾ 40 0.03 22 — — 0.99 — — PVA-7 30 0.01 0 1700 98.5 — DPMP  40 × 10⁻⁴ ¹⁾ SA: Sorbic Acid, DPMP: 2,4-diphenyl-4-methyl-1-pentene ²⁾ Content of Compound having Conjugated Double Bond based on 100 parts by mass of Ethylene-Vinyl Alcohol Copolymer ³⁾ The ethylene-vinyl alcohol copolymer thus obtained was insoluble in water.

TABLE 2 Water-Based Adhesive Aqueous Solution Aqueous Emulsion of Polymer Polymer (B) Dispersant Solid Aqueous Solid Concentration (Dispersoid) Content ¹⁾ Contents Solution/ Contents Polymer [mass %] Type [parts by mass] Type [parts by mass] [mass %] Emulsion ²⁾ [mass %] Example 1 A-1 15 — — — — — 100/0  15 Example 2³⁾ A-1 12 — — — — — 100/0  35 Example 3 A-2 12 Em-1 PVAc PVA-7 7 50 100/107 32 Example 4 A-2 12 Em-2 PVAc A-2 7 50 100/107 32 Example 5 A-2 12 Em-2 PVAc A-2 7 50 100/10  15 Example 6 A-2 12 Em-3 PVAc A-2 5 50  10/107 47 Example 7 — — Em-4 PVAc A-2 10 50  0/107 50 Example 8 A-2 12 Em-5 PVAc A-2 35 35  10/107 33 Example 9 PVA-7 12 Em-5 PVAc A-2 35 35  10/107 33 Example 10 A-2 12 Em-6 PVAc A-3 7 50 100/107 32 Example 11 A-1 12 Em-2 PVAc A-2 7 50 100/107 32 Comparative A-4 12 Em-7 PVAc A-4 35 35  10/107 33 Example 1 Comparative PVA-7 12 Em-8 PVAc A-5 35 35  10/107 33 Example 2 Comparative A-5 12 Em-1 PVAc PVA-7 7 50 100/107 32 Example 3 Comparative PVA-7 12.0 Em-1 PVAc PVA-7 7 50 100/107 32 Example 4 Water-Based Adhesive Evaluation (A) Initial Film Dynamic Water Rapid Acid [mass %] (A/B) Adhesiveness Hardness Resistance Coatability Resistance Example 1 15 100/0  C A C A A Example 2³⁾ 12 100/0  B A C A A Example 3 5.8 19/81 B A B A A Example 4 7.5 24/76 A A A A A Example 5 11.2 73/27 B A B A A Example 6 3.2  7/93 A B A A A Example 7 4.5  9/91 A C B A A Example 8 9.3 28/72 A A A A A Example 9 8.3 26/74 A B B B A Example 10 7.5 24/76 B B A B A Example 11 7.5 24/76 A A B A A Comparative 9.3 28/72 D C B C A Example 1 Comparative 8.3 26/74 A D C B B Example 2 Comparative 5.8 19/81 B C D A B Example 3 Comparative 0  0/100 C C D C B Example 4 ¹⁾ Content of Dispersant based on 100 parts by mass of Polymer (B) ²⁾ Mixing Mass Ratio of Aqueous Polymer Solution to Aqueous Emulsion ³⁾Water-based adhesive contained 30 parts by mass of calcium carbonate to 15 parts by mass of Copolymer (A-1).

DESCRIPTION OF REFERENCE NUMERALS

-   1: Polymerization Vessel -   2: Heat Exchanger -   3 to 7: Pipe -   8: Stirrer -   9: Reaction Liquid Outlet Pipe -   10: Vinyl Ester Inlet Pipe -   11, 12: Refrigerant Pipe -   13: Gas Discharge Pipe -   21: Maxblend Blade -   22: Supermix MR203 -   23: Supermix MR205 -   24: Fullzone Blade 

1. A water-based adhesive for a paper straw, comprising: an ethylene-vinyl alcohol copolymer (A); and water, wherein the ethylene-vinyl alcohol copolymer (A) has a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99.
 2. The adhesive according to claim 1, wherein the ethylene-vinyl alcohol copolymer (A) is dissolved in water.
 3. The adhesive according to claim 1, wherein the ethylene-vinyl alcohol copolymer (A) has a viscosity-average degree of polymerization from 200 to 5000 and a degree of saponification from 80 to 99.7 mol %.
 4. The adhesive according to claim 1, wherein the adhesive is an aqueous emulsion that comprises a polymer (B), as a dispersoid, comprising an unsaturated ethylenic monomer unit.
 5. The adhesive according to claim 4, wherein the polymer (B) comprises 70 mass % or more of monomer units derived from at least one selected from the group consisting of a vinyl ester monomer, a (meth)acrylic ester monomer, a styrene monomer, and a diene monomer, based on the total monomer units.
 6. The adhesive according to claim 4, wherein the aqueous emulsion comprises the ethylene-vinyl alcohol copolymer (A) as a dispersant.
 7. The adhesive according to claim 1, further comprising a polyvinyl alcohol (C).
 8. The adhesive according to claim 7, wherein the polyvinyl alcohol (C) has a viscosity-average degree of polymerization from 200 to 5000 and a degree of saponification from 80 to 99.7 mol %.
 9. The adhesive according to claim 4, wherein a mass ratio (A)/(B) of the ethylene-vinyl alcohol copolymer (A) to the polymer (B) is from 2/98 to 80/20.
 10. The adhesive according to claim 1, further comprising an inorganic filler.
 11. The adhesive according to claim 1, further comprising from 0.000001 to 0.01 parts by mass of a compound with a molecular weight of 1000 or less having a conjugated double bond based on 100 parts by mass of the ethylene-vinyl alcohol copolymer (A).
 12. A paper straw obtained by adhering a paper substrate to each other using the adhesive according to claim
 1. 13. A paper straw, comprising an ethylene-vinyl alcohol copolymer (A), wherein the ethylene-vinyl alcohol copolymer (A) has a content of ethylene units of 1 mol % or more and less than 20 mol % and a block character of the ethylene units from 0.90 to 0.99. 